Temperature control system

ABSTRACT

The purpose of the present invention is to create a temperature control system applied in heating and cooling phases of thermal processing. The invented temperature control system consists of a single frequency converter F to whose output, through contactors, are simultaneously connected: at least one heating system and at least one cooling system. The heating system incorporating resistance heating elements R is actuated by the heating system contactor SG. The cooling system incorporating gas blower, 3-phases induction motor MD is actuated by the blower motor contactor SD. The heating and cooling system contactors (SG and SD) are connected to the output of the frequency converter F and are controlled by the temperature controller RT.

The scope of the present invention is a temperature control systemapplied in heating and cooling phases of thermal processing.

In the known thermal devices a system of infinitely variable adjustmentof the power of resistance heating elements is based on a three-phasesilicon controlled rectifier (SCR) or a variable reactance transformer(VRT). An SCR connected to the electrical supply, controls the powerfollowing a control input from a programmable temperature controller,through alteration of the effective output voltage as a result ofswitching on and off full cycles of sinusoidal supply voltage(proportional-time control or group control, where mean output voltage Uis proportional to supply voltage U_(n) and switch-on time T_(z) andinversely proportional to period T; U=U_(n)xT_(z)/T), or through partialclipping of the sinusoidal supply voltage (phase control).

A variable reactance transformer (VRT) adjusts the power following acontrol input from the control system, through alteration of theeffective output current resulting from the impact upon the VRT controlcircuit current.

In standard thermal equipment the cooling process is not controlled andthe operation of the cooling gas blower motor (3-phases, inductionmotor) forcing gas circulation through the workload is not adjustable.The motor works to standard working parameters dependent on the load andis connected directly to electrical supply. In the case of motors whosepower exceeds a few dozen kW, soft-start systems are used in order toreduce the starting current (and only that). Some designs provide for atwo-speed motor featuring a 2:1 ratio of rated rotations of high and lowspeeds and, consequently, a 4:1 ratio of rated power, which enablesmotor operation at two rotational speeds and powers as well asdiversification of cooling rate: slow cooling at low speed and rapidcooling at high speed.

The latest designs call for full control of the workload cooling process(workload temperature) following an input signal from the temperaturecontroller through adjustment of the cooling blower motor rotationalspeed within 0-100% (or even more) of rated speed. This is made possibleby inverters known as frequency converter which adjust the rotationalspeed of a 3-phases induction motor by modulating the effective voltageand its frequency or phase. Performance characteristic of a blower motorprevents independent control of rotational speed, power and torque ofthe motor since a change to any of these parameters entails automaticchange of the remaining ones and, effectively, control of rotationalspeed of an induction motor results in a change of its power or torque.In such a case the motor is wired to the frequency converter's outputthrough a contactor. The frequency converter is supplied from athree-phase supply mains and transforms this energy into three-phasesupply of the motor of proper voltage-frequency characteristic, thusenforcing the requested rotational speed and output power or torque.

The purpose of the present invention is to create a single system ofalternate control of heating and cooling processes which would becontrolled by a single frequency converter.

The invented temperature control system consists of a frequencyconverter to whose output, through contactors, are simultaneouslyconnected: the heating system and the cooling system.

The heating system incorporating resistance heating elements is actuatedby the heating system contactor. The system may contain a transformerwhich can adjust voltage if required by the heating elements. Thetransformer is wired between the heating system contactor and theresistance heating elements.

The cooling system blower is actuated by the blower motor contactorwhich may also incorporate a transformer to adjust the voltage as neededby the blower motor. The heating and cooling system contactors areconnected to the output of the frequency converter and are controlled bythe temperature controller.

An advantage in this application would be to have the rated current ofthe heating and cooling systems comparable at the ratio of 0.33-3 inboth systems. Similarly, the rated supply voltage for the heating andcooling systems should be comparable at the 0.33-3 ratio of the ratedsupply voltage for both systems.

Application of a single frequency converter allows alternate control ofthe heating system and the cooling system while temperature control ison. Effectively, the heating and cooling process control is streamlinedand the number of components indispensable to provide such a system isreduced. The system significantly improves the application andexploitation properties of a thermal device.

A exemplary system for the present invention has been shown in theattached drawing. Dwg 1.

In this example the thermal device UT is a gas cooling vacuum furnacefitted with resistance heating elements R rated at 300 kW and a coolingblower powered by 250 kW motor MD.

The heating system with the transformer TR and heating elements R issupplied with rated voltage of 3×400 V, 50 Hz and rated current of 456A.

The cooling system with the transformer TRD and blower motor MD issupplied with rated voltage of 3×400 V, 50 Hz and rated current 464 A.

Both systems are wired through contactors SG and SD to the frequencyconverter F of rated output voltage 3×400 V and current 480 A and analogcontrol signal 4-20 mA corresponding to the set frequency of 0-50 Hz.

The contactors SG and SD are controlled by the temperature controllerRT.

The entire temperature control system is powered by electrical supplyESZ 3×400 V, 50 Hz with the disconnecting device LR and contactorsselected for max 630 A current.

In this configuration temperature control follows a time-temperaturesequence. The thermal process was programmed according the followingtime-temperature sequence:

-   -   Heating from ambient temperature to 800° C. at the rate of 15°        C./min    -   Soaking at 800° C. for 2 hrs    -   Heating to 1050° C. at the rate of 10° C./min    -   Soaking at 1050° C. for 1 hr    -   Cooling down to 300° C. at the rate of 10° C./min    -   Cooling down to 70° C. at the rate of 3° C./min.

Upon starting the thermal process the heating control system is turnedon. The temperature controller RT switches on the heating contactor SGand sends off a control signal to the frequency converter. The controlsignal is the result of the calculation of PID algorithm based on thevalue and error of control, i.e. the difference between temperaturereadout from temperature sensor CT and the programmed actual set value.

In the event the temperature from the CT sensor shows insufficientincrease dynamics in comparison to the actual set value, the value ofthe control signal which powers the heating elements R through thetransformer TR rises, which results in an increase of the frequencyconverter F output frequency and voltage.

Consequently, a voltage rise on the heating elements causes an increaseof their output power, which in turn boosts the temperature dynamics.

Should the temperature from the CT sensor indicate excessive increasedynamics in comparison to the actual set value, the value of the controlsignal lowers, which results in a decrease of the frequency converter Foutput frequency and voltage.

Consequently, a voltage drop on the heating elements causes lowering oftheir output power, which in turns brings about a decrease of dynamicsor even a reduction of temperature.

Following the above principles, the temperature controller RT controlsfurnace heating up to 1050° C. in accordance with a programmed processsequence.

After soaking at 1050° C. for one hour, the control system switches overfrom heating to cooling. The temperature controller RT resets thecontrol signal, turns the heating system contactor SG off and turns theblower motor contactor SD on. Then it sends to the frequency converter acontrol signal resulting from PID algorithm calculation based on theextent and change of control error, i.e. inversely than before: thedifference between the programmed actual set value and temperaturereadout from CT sensor.

In the event the temperature from the CT sensor shows insufficient dropdynamics in comparison to the actual set value, the value of the controlsignal rises, which results in an increase of the frequency converter Foutput frequency and voltage. Consequently, a frequency and voltage riseon the blower motor MD causes an increase of its rotational speed, whichin turn boosts the dynamics of temperature drop.

Should the temperature from the CT sensor indicate excessive dropdynamics in comparison to the actual set value, the value of the controlsignal lowers, which results in a decrease of the frequency converter Foutput frequency and voltage. Consequently, a frequency and voltage dropon the blower motor MD supplied via transformer TRD, causes lowering ofits rotational speed, which in turns brings about lowering of theblower's cooling capacity and causes a slower dynamics of temperaturedrop.

Following the above principles, the temperature controller RT controlsfurnace cooling down to 70° C. at which point the temperature controlsystem turns off and the thermal process is completed.

1. A temperature control system in a thermal device equipped with atemperature sensor, resistance heating elements and a motor-drivencooling gas blower, both of the latter connected through contactors tothe output of a frequency converter supplied from electrical supplynetwork through a disconnecting device, with temperature progress in thethermal device programmed on a temperature controller which controls thefrequency converter, unique in that the output of the frequencyconverter (F) is simultaneously connected through contactor (SG) to atleast one heating system incorporating resistance heating elements, andthrough contactor (SD) to at least one cooling system with blower3-phases induction motor (MD), the contactors (SG and SD) beingcontrolled by temperature controller (RT).
 2. Temperature control systemas per claim 1, unique in that between the contactor (SG) of the heatingsystem and the resistance heating elements (R) there is a transformer(TR).
 3. Temperature control system as per claim 1, unique in thatbetween the contactor (SD) of the blower and the blower motor (MD) thereis a transformer (TRD).
 4. Temperature control system as per claim 1,unique in that it is most advantageous to have the rated current of theheating and cooling systems maintain the ratio of 0.33-3.
 5. Temperaturecontrol system as per claim 1, unique in that it is most advantageous tohave the rated voltage of the heating and cooling systems maintain theratio of 0.33-3.